1. Field of the Invention
The present invention relates to a radar device for measuring the relative distance to a target by processing a signal arriving from the target as a response to an irradiated wave signal.
2. Description of the Related Art
In recent years, with the application of highly advanced information processing technologies, image processing technologies, and signal processing technologies, enhancements in performance and value added of a radar device are achieved, and thus the radar device is applied to various fields.
Among these fields, for example, in a vehicle such as an automobile or the like, for the purpose of ensuring safety when traveling or driving, the radar device is utilized to detect a relative position to other vehicles or obstacles around the vehicle in the real time.
FIG. 6 is a diagram showing an example of a configuration of a conventional radar device.
In FIG. 6, an output port of a processor 41 is connected to a feeding point of an antenna 47T via a timing generating section 42, a pulse generating section 43, an amplifying section 44, a modulating section 45, and a band-pass filter 46 which are located in a cascade manner. The feeding point of the antenna 47R is connected to an input port of the processor 41 via an amplifying section 48, a band-pass filter 49, a demodulating section 50, a low-pass filter 51, an amplifying section 52, a matched filter 53, and a timing calculating section 54 which are located in a cascade manner. A synchronized output of the timing generating section 42 is connected to a corresponding input of the timing calculating section 54. To carrier-wave inputs of the modulating section 45 and the demodulating section 50, a corresponding output of a carrier-wave generating section 55 is connected.
In the radar device having such a configuration, the timing generating section 42 generates a clock signal indicating a period matched with a range in which a distance to a target is to be determined under the control of the processor 41. The pulse generating section 43 outputs a pulse signal having a predetermined pulse width in synchronization with the clock signal. The amplifying section 44 amplifies such a pulse signal and sets the amplitude of the pulse signal to a predetermined value. The modulating section 45 interrupts the carrier wave signal (here, for simplicity, it is assumed that the frequency is 76 MHz) generated by the carrier-wave generating section 55 according to the pulse signal having such an amplitude and generates a transmitted wave signal (here, for simplicity, it is assumed that the occupied bandwidth is 24 MHz). The band-pass filter 46 suppresses spurious components attached to the transmitted wave signal and emits a transmitted wave with such spurious components having been suppressed in a direction toward the target via the antenna 47T.
When the transmitted wave is reflected by the target or when it is emitted again, the antenna 47T catches a signal arriving from the target. The band-pass filter 49 extracts an occupied band component intrinsic to the signal among the components of the signal which is supplied amplified by the amplifying section 48. The demodulating section 50 is in cooperation with the low-pass filter 51 and the amplifying section 52 subsequent thereto and detects a signal distributed over such an occupied band based on the carrier wave normally generated by the carrier-wave generating section 55 as described above by means of homodyne detection, thereby forming a base band signal.
The matched filter 53 detects a correlation of a sequence of instantaneous values between the base band signal and the pulse signal having the predetermined pulse width which is generated by the pulse generating unit 43 as described above.
The timing calculating section 54 measures a length d of a period from the time at which the above-described period is determined by the timing generating section 42 up to the instance at which the result of the correlation has the maximum value.
The processor 41 performs an arithmetic operation represented by the following equation (1) based on the length d and a propagation velocity C of the above-described transmitted wave and the signal and determines the relative distance D to the target.D=C·d/2  (1)
By the way, in such a conventional example, when components of a signal arriving from another target are included in the above-described base band signal, the relative distances to the respective targets cannot be determined with high precision.
Therefore, the pulse width W of the above-described pulse signal has to be set short drastically so that the relative distances to a plurality of adjacent targets are precisely identified.
However, the resolution Δd of such a relative distance is generally given with the following equation (2) and, between the occupied bandwidth b and the pulse width W of the above-described pulse signal, the following equation (3) is established with respect to a constant K.Δd=C·W/2  (2)W·b=K  (3)
Therefore, actually, if the bandwidth of the receiving system from the above-described antenna 47R to the output terminal of the amplifying section 52 is not set wide sufficiently, the resolution Δd is hard to be raised.
Further, in some conventional radar devices, at the instance that the instantaneous value of the above-described base band signal exceeds a predetermined threshold, the signal generated when the above-described transmitted wave is reflected by the target or when it is emitted again is identified to have arrived.
In such radar devices, due to the difference between the rising time of the above-described amplifying sections 48 and 52 and the demodulating section 50 and filtering characteristics of the band-pass filter 49 and the low-pass filter 51, the change in rising time and filtering characteristic, and the wide-ranging change in level of a signal to be received, a steep waveform of the base band signal deteriorates. For this reason, it can be highly expected that a serious error occurs in relative distance to the target.
Further, the enlargement of the above-described occupied bandwidth b may be technically performed, but, with the finite radio frequency, it is limited legally. For this reason, the occupied bandwidth b is hard to be enlarged.
Reference 1
Japanese Unexamined Patent Application Publication No. Hei 8-220214 (Abstract and paragraph 0007)
Reference 2
Japanese Unexamined Patent Application Publication No. 2002-221567 (Abstract and claims 1 to 4, and 7)
Reference 3
Japanese Unexamined Patent Application Publication No. Hei 5-223928 (claim 1)